Mounting structures for mounting a turbine wheel of a steam turbine etc., to a rotating shaft include a structure in which blades 14 of a turbine wheel are fitted one-by-one to a multiplicity of tree-shaped grooves 12 formed in an outer circumferential portion of a rotating shaft 10 as depicted in FIG. 14. FIG. 15 is an enlarged perspective view of the tree-shaped groove 12, which is symmetrical relative to a groove center S with a groove width smoothly increasing/decreasing and gradually narrowing in a groove depth direction (toward the lower side of FIG. 15) like an inverted Christmas tree such that pluralities of concave portions 18 and convex portions 20 are alternately and serially disposed on both side surfaces 16a and 16b. 
Cutting of the tree-shaped groove 12 as described above is performed by, for example, rotationally driving around an axial center and moving a formed rotary cutting tool corresponding to the shape of the tree-shaped groove 12, i.e., a so-called Christmas tree formed milling cutter relative to a workpiece in a direction orthogonal to the axial center, and burrs 22 are normally generated at the time of the cutting on an end surface of the tree-shaped groove 12 as indicated by a shaded area in FIG. 16. Since no means conventionally exist for removing the burrs 22 through a machine operation, the burrs 22 are manually removed by humans, resulting in adverse effects such as very long working hours and requirements of skills of workers. Thus, a technique is proposed that enables mechanization of rounding (chamfering) of a groove having an inclined groove surface as in the tree-shaped groove 12. For example, this corresponds to a disk parts machining device described in Patent Document 1.